Electrodeposition coating is a method wherein aqueous paint is filled in a tank and paint components are electrophoretically moved and deposited onto a surface of an object to be coated present in the tank. The main application of this electrodeposition coating is anti-corrosion coating of car bodies and it is industrially important.
In general, an electrodeposition paint contains a pigment and a pigment grinding agent. In order to introduce a pigment into paint, a pigment paste obtained by grinding the pigment in an aqueous medium at a high concentration is used as an intermediate composition. To the pigment paste, a dispersing agent is formulated to give sufficient dispersion stability to the pigment paste. The dispersing agent is required to have the characteristic to increase the dispersion stability of the pigment paste as well as performances to keep the paint stable for a prolonged period when the pigment paste is made into an electrodeposition paint by diluting it with an aqueous medium. In recent years, a cationic resin is generally used for the pigment grinding agent of electrodeposition paint.
For example, in Japanese Patent Publication Nos. 53-47143 and 54-4978, there is disclosed a cationic resin used as a grinding agent for the electrodeposition paint. Further, in Japanese Patent Laid-Open Publication No. 1-182377, there is disclosed a cationic resin which also functions as a catalyst in the curing reaction between an .alpha., .beta.-unsaturated carbonyl group and a hydroxyl group. In this reference, a cationic resin containing a tertiary sulfonium salt is disclosed, which has suitable basic for catalyst and an inhibitor effect under a corrosive atmosphere.
Furthermore, in Japanese Patent Laid-Open Publication No. 63-23919, there is disclosed a sulfonium resin which is useful as a pigment grinding and spreading agent in view of its corrosion resistance, and a cationic resin containing a tertiary sulfonium salt derived from thiodiethanol is described as a most preferred example.
However, the pigment grinding resin disclosed in this reference has a problem in that it does not provide good appearance to the resulting coated film.